Abstract The mammalian liver functions as a hub for nutrient and energy metabolism that helps maintain systemic homeostasis. Hepatic metabolism is highly responsive to physiological signals and undergoes drastic reprogramming in insulin resistant state. The liver also provides an important source of secreted proteins in circulation, including lipoproteins, coagulation factors, and endocrine factors (hepatokines). Upon release into circulation, hepatokines may act locally or on other peripheral tissues and the central nervous system to exert pleiotropic metabolic effects, as illustrated by recent studies on FGF21. Despite the expanding role of liver-derived secreted factors in systemic energy metabolism, the molecular nature and physiological action of the endocrine liver remains an important unsolved problem in molecular metabolism. Addressing this challenge provides a significant opportunity for the discovery of novel therapeutic targets and approaches for the treatment of metabolic disease. In preliminary studies, we identified Tsukushin (TSKU) as a novel liver-derived endocrine factor that exhibits markedly elevated levels in plasma from diet-induced and genetic obese mice. Genetic inactivation of this hepatokine stimulates thermogenesis and energy expenditure and protects mice from high-fat diet-induced obesity and metabolic disorders. Based on these exciting findings, we hypothesize that TSKU exacerbates diet-induced deterioration of metabolic health through attenuating thermogenesis and energy expenditure. We will test this hypothesis using a combination of in vivo and in vitro gain- and loss-of-function model systems. In Aim 1, we will examine the association of plasma TSKU levels with obesity in obese patients and establish the causative role of TSKU in diet-induced metabolic disorders. In Aim 2, we will dissect the role of TSKU in physiological regulation of adipose thermogenesis. In Aim 3, we will dissect the mechanisms through which TSKU modulates adipose sympathetic innovation using a combination of 3D imaging and molecular cellular tools. Successful completion of this highly innovative project will generate high-impact discoveries of significant scientific and translational value.